1. Field of the Invention
The present invention relates to an injection unit for making it possible to execute a high speed injection using an electric motor as well as to securely keep pressure after a metal mold is filled with a molding material, a die cast machine provided with the unit, and a motor drive injection method.
2. Description of the Related Art
Conventionally, in a die cast machine for injecting a molten metal material such as aluminum and zinc into a metal mold and molding it, an injection piston is ordinarily driven by hydraulic pressure because of a reason that the injection piston can be actuated at high speed and a structure is simple, and the like. For example, in a die cast machine disclosed in Japanese Utility Model Application Laid-Open (JP-U) No. 3-4349 (patent document 1), an injection piston is also driven using a hydraulic cylinder. However, the hydraulic cylinder is used in the die cast machine in order to prevent a situation that an apparatus is deformed or damaged since when a heating by a heater is stopped upon molding a molding material (molten material), which is expanded by cooling, the molten material in a holding furnace is solidified as well as the molten material in an injection cylinder is also solidified and expanded and an excessive force is applied to an injection piston in the injection cylinder. Accordingly, it is intended only to avoid a deformation or a damage of the apparatus but not to particularly exclude a disadvantage resulting from the use of the hydraulic cylinder.
That is, in the patent document 1, the hydraulic cylinder is disposed at a position just above the injection cylinder in the holding furnace through a frame, an attachment plate is fixedly provided at a lower end of a piston rod of the hydraulic cylinder, and a guide cylinder, in which a stepped hollow portion is formed, is fixed to a lower portion of the attachment plate, the stepped portion having a large diameter upper portion and a small diameter lower portion. An upper end flange portion of the injection piston rod is accommodated in the stepped hollow portion such that the upper end flange portion can move in the large diameter hollow portion thereof upward and downward, and a piston at the lower end of the injection piston rod is inserted into the injection cylinder such that the piston can move therein upward and downward. Further, a compression spring, which has repulsive force larger than an output of the hydraulic cylinder, is elastically interposed between the flange accommodated in the large diameter portion of the stepped hollow portion and the attachment plate. When the heating by the heater in the holding furnace is stopped on the way of molding, the molten material is solidified in the holding furnace, and the molten material in the injection cylinder is also solidified and expanded, the injection piston is moved upward against the repulsive force of the compression spring by the expansion pressure of the molten material. Since the upward movement of the injection piston at this time is buffered by the compression spring, the injection piston does not move abruptly and the apparatus is not damaged.
In contrast, Japanese Patent Application Laid-Open (JP-A) No. 6-835 (patent document 2), for example, discloses a transfer molding apparatus for manufacturing a molded product by injecting a thermosetting resin material into a metal mold by reciprocating an injection piston using an electric servo motor. The transfer molding apparatus is specifically configured such that a ball nut is directly coupled with a rotor of a servo motor so that it can be rotated thereby through a bearing with respect to a base member. A ball screw is threaded into the ball nut through balls. Further, the upper end of the ball screw is caused to pass through the servo motor, a spline groove is cut to the ball screw, and a spline nut unrotatably and fixedly disposed to a base member is threadingly attached to the spline groove. A plunger is coupled with the lower end of the ball screw through a load sensor. A fixed plate having a pot, in which a resin material is accommodated, is disposed below the plunger, and the lower end of the plunger is inserted into the pot so as to fit together with it and to be able to move upward and downward therein. A metal mold is disposed in confrontation with the fixed plate, and a movable plate moves to and from the metal mold.
According to the patent document 2, the rotation of the servo motor is converted into the reciprocating motion of the injection piston through the balls charged into a spiral groove without using a speed reduction mechanism. Accordingly, since a resin material can be accurately injected into the metal mold, and thrust force generated in a plunger is detected and fed back to the servo motor, no unreasonable force is applied to the resin material, no stress remains in the interior of a molded product, and no bubble is mixed with the molded product, and a resin has uniform orientation. Further, since the plunger is directly coupled with the servo motor without the speed reduction mechanism, the apparatus can be reduced in size, and the manufacturing cost thereof can be also reduced.
Incidentally, when a material having high solidifying speed is handled as particularly in die cast machines, an injection operation must be executed in a short time to prevent the material from solidifying on the way of injection, different from an ordinary synthetic resin material. For this purpose, the injecting motion of an injection piston for injecting an injection material into a metal mold must be executed as fast as possible. To obtain the high injection speed, a hydraulic drive system is employed in many cases because the injection piston can be actuated at high speed, in addition to that hydraulic drive system is simple in structure. Incidentally, when a case, in which an injection piston is directly reciprocated by an electric motor, is compared with a case, in which it is driven by hydraulic pressure, an injection time required by the former case is at least three times that required by the latter case. This is because since rotation acceleration is constant when the injection piston is driven by the electric motor different from the hydraulic drive, a considerable time is required until a desired injection speed is obtained after the electric servo motor starts rotation. As a result, conventionally, electric motors are not used to the injection drive of die cast machines and are only employed in special molding machines such as transfer molding machines for a synthetic resin material having a relatively slow solidifying speed as disclosed in the patent document 2.
In contrast, in the hydraulic drive system, although a required injection speed can be obtained promptly as described above, drive force cannot be accurately transmitted in many cases due to a change of viscosity of oil caused by a change of temperature thereof when, for example, an injection piston is driven, and further a working environment may be deteriorated by the oil. To cope with the above problems, it is strongly desired to drive an injection piston by an electric motor even in die cast machines. This is because the electric motor can actuate the injection piston promptly by securely transmitting the drive force thereof to the injection piston, and, at the same time, can accurately control the stroke of the injection piston, and can easily obtain an excellent working environment.